In building electrical motors and generators, insulated coils to be employed therein comprise slot portions and end portions. The slot portions fit into the radial slots disposed about the magnetic core of the rotor or stator of the electrical machine, for example an A.C. motor. A particularly satisfactory insulation for such coils comprises a mica tape, wrapped with an electrically semiconducting binding tape, both tapes being impregnated with an epoxy-styrene impregnating resin.
It is highly desirable that the binding tape, covering the mica tape, have the ability to conduct electricity, and so reduce the possibility of corona discharge between the surface of the mica tape and the radial slot of the electrical machine. In the past, filled, fibrous, semiconducting, acrylonitrile latex binding tapes have been used. This tape was effective to allow epoxy-styrene resin impregnation and curing without excessive thermal or physical degradation of the tape.
The semiconducting acrylonitrile latex tape provided a resistivity value of about 120,000 ohms/sq., after impregnation and 8 hours postcure of the epoxy-styrene impregnating resin at 150.degree. C. Such values are low enough to provide an adequate semiconducting surface that will prevent corona discharge. However, consistent uniformity in the manufacture of these tapes had been lacking. As a result resistivity values sometimes were beyond acceptable limits. Such binding tape is no longer marketed and so there is a need for suitable replacements. There is also a need for binding tapes providing lower resistivity values after varnish impregnation and cure.